1. Field of the Invention
The present invention relates to the separation and purification of para-hydroxybenzoic acid, and, more especially, to the precipitation of para-hydroxybenzoic acid from its dipotassium and/or monopotassium salt and to the purification thereof.
2. Description of the Prior Art
The dipotassium and/or monopotassium salt of parahydroxybenzoic acid may be prepared, notably, by the carboxylation of potassium phenate with carbon dioxide. The reaction is generally carried out under a pressure of a few bars (2 to 10 bars) and at a temperature on the order of 200.degree. C.
Typically, the phenate is carboxylated in the presence of free phenol and the reaction is carried out in a dispersant liquid medium. Compare, for example, French patents Nos. 1,564,997 and 2,065,098.
Upon completion of the reaction, the reaction mass is treated in order to separate therefrom the potassium salts of hydroxybenzoic and hydroxyphthalic acids obtained, in this case principally dipotassium and/or the monopotassium salt of para-hydroxybenzoic acid and, in much smaller proportions, potassium salicylate and potassium 4-hydroxyisophthalate.
This treatment may especially include addition of water, which dissolves the potassium salts of hydroxybenzoic and hydroxyphthalic acids, and a liquid/liquid extraction step using a water-immiscible solvent. Such an operation thus provides an organic solution containing, in particular, free phenol and an aqueous solution containing the potassium salts mentioned above.
Serious need exists in this art for the improved precipitation and purification of para-hydroxybenzoic acid from such aqueous solutions.
Indeed, the treatment of aqueous solutions of the dipotassium and/or the monopotassium salt of para-hydroxybenzoic acid typically includes precipitating parahydroxybenzoic acid and the other organic acids with a strong inorganic acid, and especially with sulfuric acid. This treatment requires a large excess of the strong inorganic acid and several successive washings with water to remove the maximum of the inorganic salt formed (most frequently, potassium sulfate). The para-hydroxybenzoic acid obtained can then be crystallized once, or several times, in order to reduce the level of other organic acids contained therein.
The para-hydroxybenzoic acid obtained by the usual process contains amounts of potassium sulfate, and to a lesser extent, salicylic acid and 4-hydroxyisophthalic acid, which are still too high for certain applications of para-hydroxybenzoic acid in the food industry.